Providing this information as plain text was doubtless worth the trouble
ten years ago, but it seems likely that hardly anyone reads it in this
format anymore.  And the effort required to maintain these files (in the
form of extra-complex markup rules in the relevant parts of the SGML
documentation) is significant.  So, let's stop doing that and rely solely
on the other documentation formats.

Per discussion, the plain-text INSTALL instructions might still be worth
their keep, so we continue to generate that file.

Rather than remove HISTORY and src/test/regress/README from distribution
tarballs entirely, replace them with simple stub files that tell the reader
where to find the relevant documentation.  This is mainly to avoid possibly
breaking packaging recipes that expect these files to exist.

Back-patch to all supported branches, because simplifying the markup
requirements for release notes won't help much unless we do it in all
branches.

Per report from Colin 't Hart

used to work as intended, but got broken some time ago (a quoted empty string
is not an empty string), and got broken some more by the changes to generate
ecpg's preproc.y automatically.  Given all the unprotected uses of $(PERL)
elsewhere, it seems best to make use of the $(missing) script rather than
trying to ensure each such use is protected individually.  Also fix various
bits of documentation that omitted to mention Perl as a requirement for
building from a CVS pull.  Per a complaint from Robert Haas.

Add README.CVS to help CVS folks find this information.